This invention is related to removing thermal barrier coatings from surfaces. More particularly, the invention is related to removing thermal barrier coatings without attacking underlying coatings, such as nickel aluminide or platinum nickel aluminide.
Higher operating temperatures increase the efficiency of gas turbine engines. Advanced propulsion technology now involves the operation of engines at temperatures of 1000.degree. C. and above. Such high operating temperatures challenge the durability of engine components. The use of nickel and cobalt-base superalloys has provided some advance in high temperature durability. However in certain sections of a gas turbine engine, such as in the turbine, combustor and augmentor, the metallic components must be coated with a thermal insulator to provide further high temperature operation durability.
Thermal barrier coatings (TBC) are typically used on metallic components to provide the further high temperature operation durability required in many sections of the modern turbine engines. Thermal barrier coatings typically include a ceramic coating deposited on a metallic bond layer that is deposited on the metallic component surface. Typically the bond layer is platinum nickel aluminide, nickel aluminide, aluminided MCrAlY (where M equals nickel, cobalt, nickel-cobalt and iron; Cr is chromium; Al is aluminum; Y is yttrium) or mixtures thereof. The ceramic coating can be some form of stabilized zirconia, such as zirconia stabilized with calcia, hafnia, magnesia, yttria or any of the rare earth oxides, which form a refractory oxide thermal barrier coating. The thermal barrier coating has a very low thermal conductivity, low density and high melting point.
During service life, coated metallic components of engines for commercial aircraft, some military aircraft and for power generation, crack, spall or undergo chemical and physical attack. Hence, periodically these engines must be overhauled. During overhaul, turbine blades and vanes that have not exceeded creep limits and are not otherwise severely eroded or damaged can be refurbished for reuse. Coatings, such as the thermal barrier coating and bond coat, are stripped from the metallic component and the component is reworked and cleaned, recoated and returned to service. The thermal barrier coating must be completely removed before recoating the metallic surface with fresh bond layer and thermal barrier coating.
Ault, U.S. Pat. 4,439,241 discloses cleaning a turbine part, repairing the part, e.g., by welding or straightening, and reapplying the external coating. In the process, a part, such as a turbine blade, is disposed in a holder in an autoclave and is subjected to the flow of an alkaline solution of potassium hydroxide (KOH) or sodium hydroxide (NaOH) and water at a temperature between 150.degree. C. to 235.degree. C., at a pressure between 150 psi to 350 psi. The potassium hydroxide solution is 10-45 weight percent.
First stage turbine blades experience a build up of silicon-calcium (Si-Ca) rich dirt and debris during engine service that inhibits the flow of cooling air and creates hot spots, which lead to accelerated internal corrosion that may cause through-wall material failure. Previous autoclave processes have been ineffective at removing large amounts of this internal debris. Irvine et al, New Method for Turbine Blade Internal Cleaning, a paper presented at the Internal Gas Turbine and Aeroengine Congress and Exposition, Houston, Texas, June 5-8, 1995, proposes pumping heated caustic at pressures approaching 400 psi through the internal cavities of turbine blades. The increased caustic flow from this process, compared to previous autoclave processes, improves cleaning effectiveness. The process reduces repair costs by improving part yield during repair.
The caustic process can be used to clean engine parts such as air foils and can also be used to remove ceramic cores from airfoil castings. The caustic reacts with oxides and hydroxides to form salts of the alkali metals which are soluble in water. This chemical reaction makes it possible to clean airfoils and to remove ceramic cores. Additionally, the chemical reaction of the caustic process has been proposed to remove thermal barrier coatings by dissolving the alumina interface between the thermal barrier coating and the underling metallic component. Unfortunately under the conditions taught in the art, the caustic process also attacks the nickel aluminide or platinum nickel aluminide of the underlying metallic component. When the metallic component is subsequently cleaned and recoated with thermal barrier coating and heat treated, a black layer believed to be mostly nickel oxide, peels from the airfoil internal and external surface. Thus, there is a need to provide a caustic process to remove thermal barrier coatings from an underlying metallic component without forming the undesired black peel (flakes). The present invention is based on the discovery that the peel layer (flakes) may be avoided by conducting the caustic process under selected processing conditions.